1. Field of the Invention
The present invention relates to a damper of a type with a variable damping force which is built in a suspension system of a vehicle such as a motor vehicle.
2. Description of the Related Art
A damper which is built in a suspension system of a vehicle is provided with a cylindrical damper main body, and a rod which is inserted into the damper main body so as to be relatively movable in an axial direction. One of the damper main body and the rod is connected to an unsprung member and the other of the damper main body and the rod is connected to a sprung member.
As the kind of dampers, there can be listed a hydraulic damper which has that oil chamber inside a damper main body into which a piston to be connected to a rod is inserted, and a friction damper which is arranged that a rod is fit into a damper main body to thereby frictionally contact the two members together.
In the hydraulic damper, an orifice is formed in the piston to generate a damping force by a flow resistance of oil which flows through the orifice. Conventionally, the damping force is varied by changing the diameter of the orifice.
In the friction damper, on the other hand, as disclosed in Japanese Published Unexamined Utility Model Registration Application No. 84447/1991, the following arrangement is also known. Namely, there is mounted on a damper main body an actuator which is made up of a magnet coil with a winding axial line (i.e., an axial line around which the magnet coil is wound) which is at right angles to an axial line of the damper, and a piston which comprises a magnet to be urged towards a rod upon charging of electricity (or by electric charging) to the magnet coil.
FIG. 10 is a graph showing a change in the correlation between the moving speed of the rod and the damping force depending on the orifice diameter in the hydraulic damper. In the graph, line "a" shows the change in case where the orifice diameter is large, and line "b" shows the change in case where the orifice diameter is small. As can be clearly seen in this graph, in a region in which the moving speed of the rod is small, the damping force does not change so much even if the orifice diameter is changed. Here, the moving speed of the rod which occurs at the time of normal running of the vehicle falls within a low-speed region as shown by "A" in FIG. 10. Therefore, in the conventional system in which the orifice diameter is changed, it is not easy to effectively change the damping force of the hydraulic damper at the time of normal running of the vehicle.
Further, in the friction damper described in the above-described Japanese Published Unexamined Utility Model Registration Application No. 84447/1991, due to the fact that the direction of the winding axial line of the magnet coil lies in the direction at right angles to the axial line of the damper, the actuator protrudes outwards of the damper main body. It has therefore a disadvantage in that the damper becomes large in size.
In view of the above-described points, the present invention has an object of providing a damper of a type with a variable damping force which, without enlarging the damper, can effectively change the damping force even in the region in which the moving speed of the rod is small.